Despite outlining the most ambitious job-shedding program in 15 years, the Abbott government has assumed most staff will leave of their own accord without seeking a payout.

The funding shortfall will easily exceed half a billion dollars over three years if the costs incurred in recent years are any guide.

The budget papers show the government believes it will spend $273 million in 2013-14 on separation payments for staff - the highest sum ever spent in a year. This record redundancy bill coincides with a net loss of 2695 full-time civilian jobs and 85 military positions.

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Yet while the government plans to shed more than three times as many civilian jobs in 2014-15 - 8200 full-time staff - it has only set aside $105 million for payouts.

Its forecast payments then fall to a relatively tiny $49 million by 2017-18, despite plans to shed another 5000 jobs that year.

The Coalition has regularly attacked the former Labor government's failure to fund public service redunancies.

Shortly after winning office last year, the Abbott government abandoned its pledge to shed 12,000 staff without resorting to retrenchments, blaming what it called Labor's "hidden job cuts and unfunded redundancies".

Finance Minister Mathias Cormann said in November the Gillard and Rudd governments' economy drives had forced agencies to shed staff without funding the resulting payouts, pushing them into operating losses.

This week, Mr Cormann and Public Service Minister Eric Abetz said they were managing "Labor's largely indiscriminate cuts in a more structured manner, including funding the resulting redundancy payments, to ensure that the operations of agencies is not compromised".

However, Labor's assistant economic spokesman, Andrew Leigh, said on Thursday it was "bizarre to think that the exit rate in the public service is going to slow".

"What this illustrates is that they [the government] have not thought through the implications of these public service job losses," Dr Leigh said.

"They're hoping to get by with a lower number of public servants per head of population than existed towards the end of the Howard government, but they'll quickly realise they can't do much without them and without the services and advice they provide."

When asked why estimated future spending on redundancies was so low, a Finance Department spokeswoman said that, for job losses in 2014-15, "in some cases agencies may meet these costs from within existing resources".

"The process for accessing funding from this pool for 2014-15 has not yet commenced and therefore funding is yet to be allocated or appropriated."

The federal bureaucracy's natural attrition rate - the proportion of staff who voluntarily leave their job each year - has collapsed recently, falling to 3.8 per cent from a high of 6.9 per cent in 2008.